1. Field of the Invention
Embodiments of the invention relate to an organic light emitting display, and more particularly to an organic light emitting display and a degradation compensation method thereof capable of compensating for degradation of an organic light emitting diode.
2. Discussion of the Related Art
An organic light emitting display, which has been considered as the next generation display, includes a self-emitting element capable of emitting light by itself, and thus has advantages including a fast response time, a high light emitting efficiency, a high luminance, a wide viewing angle, etc.
The organic light emitting display includes an organic light emitting diode (hereinafter, abbreviated to “OLED”) serving as the self-emitting element. The OLED includes an anode electrode, a cathode electrode, and an organic compound layer formed between the anode electrode and the cathode electrode. The organic compound layer includes a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, and an electron injection layer. When a driving voltage is applied to the anode electrode and the cathode electrode, holes passing through the hole transport layer and electrons passing through the electron transport layer move to the light emitting layer to form excitons. As a result, the light emitting layer generates visible light.
In the organic light emitting display, pixels each including the OLED are arranged in a matrix form, and brightness of the pixels is controlled based on a gray level of video data. The organic light emitting display is mainly classified into a passive matrix organic light emitting display and an active matrix organic light emitting display using thin film transistors (TFTs) as a switching element. The active matrix organic light emitting display selectively turns on the TFT serving as the active element to select the pixel and holds the light emission of the pixel using a hold voltage of a storage capacitor.
There are several factors which reduce the luminance uniformity between the pixels in the organic light emitting display. A deviation between electrical characteristics of driving TFTs of the pixels, a deviation between cell driving voltages of the pixels, a degradation deviation between the OLEDs of the pixels, etc. have been known as the factors. The degradation deviation between the OLEDs of the pixels is generated because the pixels each have a different degradation speed based on the same usage time. The degradation deviation between the OLEDs leads to an image sticking phenomenon, thereby reducing image quality of the organic light emitting display.
To compensate for a luminance reduction resulting from the degradation of the OLED, a technology which applies a uniform programming current to the OLED to thereby sense a threshold voltage of the OLED and differently adjusts video data for the light emission of the OLED based on the sensed threshold voltage, has been known. As the degradation of the OLED deepens, the sensed threshold voltage increases and an output luminance is reduced. Therefore, a related art technology sets a compensation target for the luminance compensation and modulates the video data based on the sensed threshold voltage, thereby adjusting the output luminance in conformity with the compensation target.
However, as shown in FIG. 1, in the related art technology, the compensation target is set to an ideal luminance of an OLED, which is hardly used (i.e., has not yet been degraded), and the degraded pixels are compensated for their luminances based on the compensation target. Therefore, as usage time of the OLED passed, a luminance gap between the compensation target and a luminance to be compensated gradually increases. Hence, in the related art technology, as usage time of the OLED passed, power consumption required to compensate for the degradation of the OLED gradually increases. In FIG. 1, ‘Best Pixel’ indicates a pixel showing the ideal luminance, and ‘Worst Pixel’ indicates a pixel which is degraded and is subject to compensation as usage time of the OLED passed.
Furthermore, in the related art technology, because the compensation target is set to the ideal luminance, the luminance gap between the compensation target and the luminance subject to compensation gradually increases as usage time of the OLED passed. Hence, a compensation error increases. One factor generating the compensation error is an IR drop resulting from a resistance difference of a cell driving voltage supply line based on its location. As the compensation error increases, a luminance balance and a color balance of a display image of the organic light emitting display may not be kept.